1. Field of the Invention
The present invention relates to a vacuum apparatus for multiple-bed industrial hide driers, and to a drier that includes the apparatus.
2. Description of the Related Art
It is known that the vacuum method of drying hides has, with respect to conventional methods that entail laying the hides on frames placed in ventilated and dehumidified rooms or in heated tunnels, the advantage of a hide that is smoother and flatter and also better adaptable to the subsequent finishing operations. Furthermore it allows shorter drying times, on the order of minutes instead of hours, and lower labor costs. Finally, modern driers entail lower energy consumption and occupy far less space than barometric-pressure systems.
However, vacuum driers have the drawback of higher initial and running costs and of lower final hide quality, especially for thinner hides with finer grain, due to the considerable heat-induced stresses on the hides and to the depletion of fatty materials contained in their fibers. In vacuum driers, the hides are in fact arranged on beds which are heated to approximately 60-80.degree. C., i.e. to temperatures that are considerably higher than the "body" temperature of the animals from which they are obtained, in order to make the residual moisture evaporate as quickly as possible. In order to lower the evaporation temperature, the beds are hermetically enclosed by airtight lids so as to form a series of evaporation chambers that are connected to a vacuum pump by means of a circuit along which one or more condensers and/or condensate separators are arranged as shown in FR-A-2557888. The vapor state diagram clearly shows that the higher the vacuum produced in the sealed chambers, the lower the evaporation temperature at equilibrium.
For this purpose a high vacuum is produced in the vapor circuit by means of vacuum pumps of the liquid-ring type or of the variable-chamber cylinder type, using water or oil as working fluids; these pumps can produce vacuum up to 95% with residual absolute pressures of less than 30 mbar.
In practice, the pressure is gradually reduced down to a vacuum of more than 90%, which corresponds to an absolute pressure of approximately 80 mbar, with a vapor equilibrium temperature of approximately 45.degree. C. Accordingly, the beds are heated to a temperature of at least 60-70.degree. C. to produce a thermal gradient that allows evaporation of the residual moisture of the hides in an acceptable time. In order to reduce the evaporation time it is obviously possible to increase the temperature of the beds to more than 80.degree. C. so as to increase the thermal gradient and thus the transfer of heat toward the hides, but this entails the risk of creating irreparable damage to their fibers.
Furthermore, in these conditions, and by cooling the condensers with water at a temperature of approximately 15.degree. C., it is possible to dry the hides to a residual moisture content of approximately 30%. In order to provide more intense drying it is necessary to extend the time for which the hides remain on the beds, negatively affecting both the productivity of the apparatus and the quality of the dried product.
U.S. Pat. No. 3,027,651 describes an apparatus for removing condensable vapors including a Roots-type compressor arranged upstream of of a two-step condenser and of a gas ballast press. The compressor acts on the vapors as a supercharger to a low-temperature condenser unit operating between -20.degree. C. and -40.degree. C.